Magnetic core



May 23, 1933. A. o. AUSTIN 1,910,227

MAGNETI C CORE Filed Jan. 5. 1951 mui 35 /0- /N VENTO/e 36 BY CQ M 4 Uy ATTORNEY 7 UNITED STATES Patented May 23, 1933 PATENT oFFicE ARTHUB 0. AUSTIN, F NEAR BABBERTON, OHIO, ASSIGNOR, BY MESNE ASSIGNMENTS,

T0 THEQHIO BRASS COMPANY, 0F

JERSEY MANSFIELD, OHIO, A. CORPORATION OF NEW MAGNETIC com'.l

A. Applicationi led January 3, 1931. SerialNo. 506,380.

will be strong and eilicient. d

Other objects vand advantages will appear Y from the following descri tion.

The invention is exemp ified by the combination and arrangement of parts shown 1 n the accompanying drawing and described in the .following specification, particularly pointed out in claims.

In the drawing:

Fig. 1 is a fragmentary longitudinal section of a portion of a core showing one embodiment of the invention.

Fig. 2 is a plan of a core showing another modification. y

Fig. 3 is a fragm ntary section on line 3-3 of Fig. 2. i i

Fig. 4 is a plan of a core showing a differthe appended i ent modification.

' Fig. 5 is a section on line 5-5 of Fig. 4.

Fig. 6 is a view similar to Fig. t", showing y another modification of the invention.

Fi 7 is a viewv similar to Fig. 2 showing a diiigerent form of the invention. Fi s. 8 and 9 are sectional views showing details.

Fig. 10 is a section on an enlarged scale of a portion of Fig. 7.

' vIn the manufacture of current transformers, it has heretofore been customary t o use circular sheet metal punchings from which to build up the core. Transformers with cores made of these punchings are not only costly but in some instances do not have satisfactory electrical characteristics.

. The present invention is especially applicable to the magnetic core for a current transformer, such as that of my prior Patent No. 1,723,000, in which the primary makes one or more turns around the core. The eiective lines of force not only travel through the iron but through an air path as well. Where the core of'this type is used in bushings as shown in my patent referred to above, it is desirable to obtain mechanical strength and successive turns.

and it is more 11 rigidity in the core. This can be obtained by wrapping or rolling the core of suitable 'sheet iron. By this manner of construction, a short-circuit turn will be created in the core if electrical contact is made between It is therefore, necessary to preventa shortcircuit turn by interposing insulation between the layers of sheet metal as shown at 10 in Fig. 10. The insulation may be paper orV other suitable material interposed between the layers of 4sheet iron If several sheets 11 are wound together,with two or more layers 10 of insulation in the group, an electrical break-down will have to take place through all of the insulating sheets before a short-circuit turn will result. It is, therefore, advisable to wind several sheets at the same time as the ratio of magnetic material to insulation may beincreased but in addition tlie probability of a break-down causing a short-circuit turn will be greatly reduced. This method of construction makes it possible to use fewer layers of insulation so that greater latitude is permitted in the insulation itself. By using an insulation such as asbestos or mica, which will withstand heat, the core may be annealed after wraping. 1

In order to reduce eddy currents, the several sheets may be coated with a varnish which contains a residue or material which will provide suicient insulation to reduce eddy currents after the baking out. A material which will readily stand annealing temperature may be provided by the introduction of finely -ground earth or silica into the var nish or binder used for holding the material during the forming operation.

The core may be made in various ways, as from bundles of magnetic wire or rods, as well as from sheets. After the core is formed, it may be annealed and then impregnated with a suitable oil, varnish or enamel and then baked or cured. The hardening of the coating material, together with the arrangement, will provide a very solid core which will give the necessary rigidity for the support of the primary winding and insulating 00 bailles when desired.

ioA

Since an ap reciable part of the magnetic path is throug the air, it is evident that the ma etic core may be made up in sections if desired without seriously affecting the magnetic reluctance of the circuit. 'Ihe core may be built up by winding stri s of magnetic'material in a plurality of he 'cal la ers in any suitable form. The strips may oli'set in the different layers, as shown at 20 Fi 1.

layer of insulation 14 may be formuf on the outside of the core in any suitable way as by winding with varnished cambric. By staggering the turns in different layers, as shown in Fig. 1, a stron mechanical construction is obtained an a lon magnetic core may be provided with a ra er narrow width of sheet 20. Owing to the stiffness of the sheets, it is much easier to wind narrow sheets than wide ones. By heating the sheets, as they are wrapped about the core, they will contract upon cooling and maintain a very strong mechanical construction. By maintaining a high temperature of the sheet as it is rolled in place, stiffness in the sheet is reduced and the hardening effect due to the rolling is greatly reduced. With this method, the ratio of magnetic material to insulation may be materially increased as the air spaces will be greatly reduced.

After the wrapping operation, the sheets comprising the core are clamped or bound in position until after the annealing and impregnating operations have been completed, so as to prevent loosenin of the sheets. Where itis desired to obtain a particularly tight construction, the center portion is maintained vat a lower temperature during the winding operation so as to provide a substantially smaller core during the forming of the hot sheet. This cooling may be applied to the sheet being wra ped into position after it is partially woun The cooling ma come about naturally or may be brought about by any suitable cooling means such as an air blast or the application of a cooled surface. Where the cores are wound u on tubular members, the cores may be rea ily kept cool by the gssage of air or liquid through the tubes.

e wrapped type of construction which I have devised for transformer cores oifers material advantages for magnetic circuits of various types, as the cores may be readily made `from strip or sheet material, frequently,

without loss. nated.

In the form of the invention shown in Figs. 2 and 3, the sheet metal 27 may be wound as explained above. The magnetic material may be wound with but a single turn between adjacent layers of insulating material or several layers of magnetic material may be placed together with one or more insulating zones separating the several stacks of magnetic material as desired. The usual method 1s to wrap the various sheets `with their insu- Expensive dies are also elimilation upon a mandrel. With this type of magnetic core, the magnetic flux has a path largely throu h the magnetic material but also through t e insulating layer. Since the area for the air path is very large, the reluctance of the magnetic circuit may be made low by maintainm a small thickness for the insulation. The c aracteristics of this construction can be used to advantage in some cases as the permeability of the magnetic path is different from that where the path is entirely of iron or magnetic material. Where it is desired, the ma etic reluctance can be further reduced by cin the magnetic clips 28 over the ends o the sheets, as shown in the drawing, which provides a path between the inner and outer turns. The magnetic circuit provided by the clips may be formed in several different ways, providing, however, a short-circuit turn is not formed.

In place of the magnetic clips 28, a ribbon 29 may be usedv for wrapping the core, as shown in Figs. 4 and 5 or cup shape rings 30 which do not set u a short-circuit turn may be used, as shown 1n Fig. 6. If desired, several sets of magnetic clips or return paths may be'used at intermediate points in the winding. Where a ribbon winding is used it should be provided with one or more insulating joints as shown at 32 and 33, to prevent the ribbon from forming a closed electrical conductor. Since the voltage across the joint is not very high, the ends of the magnetic ribbon may be brought close together or overlapped so as to permit ready passage of the magnetic flux across the insulation.

The ty of cores just described may be used to a vantage where the cores are desired for transformers having appreciable outputs as it may be desirable 1n such cases to g'lpvide cooling ducts in 4cores of this kind.

e increased space due to the cooling duct tends to increase materially the reluctance of the magnetic circuit unless the magnetic cli s or by-passes are used. These clips can Ee used not only to provide necessary separation for the cooling passages but to reduce the reluctance of the magnetic circuit. Several cores having different diameters may be placed together concentrically with suflicient space between to provide the cooling slots as shown at 31 in Fig. 7 or, if desired, the return magnetic path may be interposed at different points in the magnetic sheets as they are wound up.

By the use of the magnetic clips as illust-rated in the drawing, the magnetic characteristics of the current transformer may be materially varied, owing to the'difl'erences in length of magnetic paths, particularly where there is considerable difference between the inner and outer diameters of the core and t-hc return magnetic paths are placed intermediate the inner and outer surfaces of the core. This is due to the fact that a given flux will be developed with a much less magnetomotive force near the center of the core where the effective length ofthe magnetic path is short than atthe outer edge where the path is con- 8 siderably longer. This will tend to smooth out irregularities due to the saturation characteristic of the iron. This method is used to advantage in the construction of seriesor shunt impedances for the correction of the current transformer characteristics, as disclosed in my previous application Serial Number 433,921.

The method of construction comprising the winding of several sheets with insulation between groups, together with a return magnetic path, can be used to advantage in the production of power and potential transformers as the construction results in a strong magnetic core of low cost. The magnetic core may be of any desired shape having, for instance, straight sides and oval ends or it may be rectangular in shape. By heating the iron or magnetic material as it is wound or wrapped in place, the contraction upon cooling will tend to cause a dense core of good mechanical strength. If the magnetic properties of the material are disturbed during the winding or wrapping operation, these may be materially corrected by subsequent annealing as previously described. The

whole may be wrapped upon a core or hollow v spool which can remain in position. yIn genl eral, however,

this spool should be made of a low resistance or non-magnetic material to cut down possible eddy currents. In some cases, by using a suitable hollow core mandrel, the magnetic circuit may be wound through coils which have been previously formed. This obviates the necesslt of winding the coils after the core has been formed. In many cases, however, the coils may be readily wound or suitable spools or cores placed over the magnetic circuit, the equipment being similar in action to that used in taping or wrapping insulation, or in applying insulation to rings.

Any of the several types of cores shown may be used for reactance coils having a wide variation in the reluctance of the magnetic circuit. When the cores are used for reactance coils in which it is desired to incorporate the characteristics of an air gap in the magnetic circuit, a suitable winding is placed through and around the magnetic material. When the core is used for a current transformer in a bushing, the transformer winding is disposed around the axis of the core but when it is used for a reactance coil, the turns are in a plane parallel to the axis of the core and threaded through the opening in the core. This type of construction may also be used for a core in a transformer where it is desired to decrease the reluctance of the magnetic circuit, giving the core characteristics like that of a magnetic circuit containing an air gap.

With the construction shown, it is possible to make a strong magnetic core in which the reluctance of the magnetic circuit may b e varied over a large range. At the same tlme, circular or rectangular cores may be formed with a loss of little or no material as the cores may be made from strip material. lWhere the cores are used for reactance forming the equivalent of a closed magnetic circult, the type of construction makes it possible to change the magnetic flux characteristics in the core very materially.

Where the inner diameter ot'y the core is small compared to the outer diameter, the shorter magnetic path adjacent the center will permit more liux for a given number of ampere turns than the longer path formed by the outer layers. By varying the inner and outer diameters, or the msulation between turns, or the magnetic shunts 34, 35 and 36, lFig. 7, the lines of force set up for a given magnetizing force may be made to vary. If

it is ,desired to cause an early saturation in a portlon of the core and a delayed saturation i in another portion of the core, this is readily brought about by properly regulatin the factors just' named. This is illustrated by Fig. 7. By using insulation between layers of magnetic material and using shunt magnetic paths 35, together with a shorter length of magnetic path, a given number of ampere turns affecting the core would build up a hlgher flux per given area in the inner portion of the core than in the outer layers, particularly if part or all of the shunt paths 34 and 36 are removed and the insulation between turns increased.

With the construction shown, it is possible to give improved'magnetic characteristics for adjusting impedance or reactance coils used for correcting phase angle, ratio of current transformers or in making current limiting reactances. The type of construction makes it possible to build a current limiting reactance of high mechanical strength and good insulation for limiting the short-circuit currents in buses'or feeder circuits. High mechanical strength with the desirable magnetic.

feeder circuits `or in the neutrals of transy former banks.

I claim:

1. A magnetic core for electrical apparatus comprising a sheet of magnetic material wound in convolutions, and a retainer of magnetic material, spanning the edges of a plurality of convolutions at the end of said core and in contact with said edges for holding said convolutions together, said retainer having a radially extending portion forming a return path for magnetic lines of force.

2. A magnetic core` for electrical apparatus comprising a sheet of magnetic material wound to form a plurality of convolutions certain of said convolutions being spaced apart providing air s aces therebetween.

3. A magnetic core or electrical apparatus comprising a sheet of magnetic material wound to form a plurality of convolutions,

, and holding members for securing said conj volutions together, said holding members having portions thereof disposed between ce r tain of said convolutions and separating said convolutions to form air spaces therebetween.

4. The method of forming a magnetic core for electrical apparatus com rising thesteps of winding or wrappin s eet' material to form a plurality of convo utions, securing the convolutions together, annealing the core thus formed,- impregnating the core with insulating material and curing or baking the 1mpregnated core. l

5. The method of forming a magnetic core comprising the steps of wrapping or winding sheet material together while heated and permitting the core thus formed to cool to cause the parts to contract. 'f

6. The method of forming a magnetic core comprising the steps of winding or wrapping sheet material together, the outer wrapper being heated during the forming operation While the inner portion of the core is cooled, the core being permitted to cool after forming so that the outer wrappings will contract upon the inner portion of the core.

7. A magnetic core comprising a lurality of thin sheets of magnetic materia? laid together and wound upon a common axis, the inner ed of adjacent sheets being circumferential y spaced from each other.

8. A magnetic core comprising a plurality of layers of magnetic sheet material encircling a common axis, and insulation disposed between some of said la ers while other layers are in direct electrica and mechanical contact.

9. A magnetic core comprising a plurality of layers of magnetic sheet material wound about a common axis, said material being disposed in groups each comprising a plurality of sheets placed together in contact with one another, and insulating material separating said groups.

10. A magnetic core comprising a plurality of layers of magnetic sheet material wound about a common axis, insulating material seiarating some of said layers and some of sai layers being spaced apart to provide air passages between said layers.

11. A magnetic core comprising a plurality of sheets of magnetic material laid together to form a stack and wound about i common axis, the inner ed I s of adjacent sheets being circumferentiall; spacedfrom each other, and insulatin material separating the convolutions of said stack.

12. A magnet-ic core comprising a plurality of layers of magnetic' sheet material wound upon a common axis, some of said layers being spaced apart to provide air passages between said layers, and magnetic material disposed radially at the end of said core to form a return path for magnetic ilux radially overlapping the end edges of the layers of magnetic material forming said core.

13. A ma etic core comprisinga plurality of layers o magnetic sheet material wound about a common axis, clips of magnetic material extending radially of said core at the ends thereof and overlappin some of said layers, the ends of said clips ing extended between some of said layers to space said layers apart.

14. A ma etic core formed of a plurality of layers ofgldiagnetic sheet material wound about a common axis, certain layers of said core being spaced apart to provide air passages between said layers and magnetic material bridging the space between said layers to provide a return path for magnetic flux across said space.

15. Ama etic core comprisinga plurality of sheets oilnagnetic material laid together and wound about a common axis, the inner edges of adjacent sheets being circumferentially spaced from each other, insulation separating the layers of the core thus formed into groups having a lurality of layers in each grou some of t e adjacent layers of said core space therebetween, and magnetic material disposed radially of said core adjacent the en edges of said layers and forming a return path for magnetic flux.

16. A ma etic core for electrical apparatus comprising a plurality of superimposed layers of magnetic material, each layer includin a strip of said material wound helically a ut an axis, the joints between adjacent convolutions in each layer bein staggered relative to the joints in the adjacent ayers. e

17. A magnetic core for electrical apparatus comprising magnetic material in strip form and helically wound about an axis in a plurality of layers, the joints between the convolutions in adjacent layers bein sta gered, and binding material for holding t e convolutions together.

18. A laminated magnetic core for electrical apparatus, and means for decreasing the reluctance of the path of the magnetic flux in said core comprising a strip of magnetic material extending transverse to the laminations of the core and in a direction to form a return path outside of said laminations for the ux in the magnetic circuit of the core.

19. A laminated magnetic core having adjacent laminations thereof insulated from one another, and means for decreasin the reluctance of the magnetic path of sai core comprising a strip o` magnetic material exing spaced apart providing air vtending transverse to the laminations of the core and in a direction toform a return path outside of said laminations for the flux in the magnetic circuit of the core.

20. A4 laminated core for electrical ap aratus having adjacent laminations insu ated from one another, and means for holding said laminations together and for decreasing the reluctance ofthe magnetic circuit in said core, said means comprising a clip of magnetic material extending transverse to the laminations at the edges thereof and having portions holding said laminations together, said clip having contact with said laminations and extending in a direction to form a return path outside of said laminatons for the flux in the magnetic circuit oi the core.

In testimony whereof I have signed my name to this speciicationfthis 2nd day of January A. D. 1931.

ARTHUR O. AUSTIN. 

